1. Filed on the Invention
The present invention relates to a method for magneto-optical recordation and reproduction, and an apparatus therefor.
2. Related Background Art
Magneto-optical mediums are known, as a reloadable high density recording system, which record information by writing magnetic domains on a magnetic thin film with thermal energy of a semiconductor laser and reading out the information by utilizing magneto-optical effect. In recent years, the magneto-optical medium is required to record information in a still higher recording density for larger recording capacity.
The linear recording density of an optical disk such as magneto-optical mediums depends largely on the laser wavelength and the numerical aperture NA of the objective lens of the optical reproduction system. The beam waist diameter depends on the laser beam wavelength and the numerical aperture NA of the objective lens of the optical regeneration system, so that the detectable spatial frequency for record mark reproduction is limited approximately to 2NA/.lambda.. Therefore, for higher density of the conventional optical disks, the laser wavelength of the reproducing optical system should be shorter and the NA of the objective lens should be larger. However, improvements in the laser wavelength and in the numerical aperture of the objective lens are naturally limited. For further improvement of the recording density, techniques are being developed to improve a constitution of the recording medium and the method of readout.
For example, Japanese Patent Application Laid-Open No. 8-7350, discloses a magneto-optical recording medium and a magneto-optical recording-reproducing apparatus that use a domain-enlarging reproduction system. In this reproduction system, a magneto-optical disk has an exchange-coupled three-layered magnetic film as the memory layer; magnetic domains in the memory layer are transferred to the readout layer which is heated with a light spot to lower the coercivity force thereof; the transferred domains are enlarged by application of a reproducing magnetic field; and the information is reproduced by utilizing disappearance of the transferred and amplified domain by application of a reversed reproducing magnetic field. The process disclosed in the above laid-open publication is said to improve greatly the signal-to-noise ratio (S/N) of the reproduced signals of fine domains to enable a higher recording density.
The constitution of the magneto-optical recording reproduction apparatus of the domain-enlarging reproduction system is explained below, with reference to FIG. 1 showing the constitution of the apparatus. In FIG. 1, magneto-optical disk 1 comprises a sample-servo substrate 2, a magneto-optical medium 3 coating the substrate, and a protection layer 4, where magneto-optical medium 3 is constituted of a memory layer and a readout layer. The apparatus has an objective lens 6 as the condenser lens, an actuator 5 for driving condenser lens 6 for focusing and tracking, a semiconductor laser 7 for projecting a light beam, a collimator lens 9 for collimating the light beam, a beam splitter 12 for separating the light beam, a .lambda./2 plate 14, a polarized beam splitter 15, photosensors 17 for photoelectric conversion, and condenser lenses 16 for photosensors 17.
Differential amplification circuit 18 amplifies differentially signals condensed and detected in the respective polarization directions of the light beam. Addition amplification circuit 19 adds and amplifies the signals condensed and detected in the respective polarization directions of the light beam. Magnetic head 26 applies, in recordation, a modulated magnetic field in accordance with signals to be recorded onto the laser-projected spot on magneto-optical disk 1, and applies, in reproduction, an alternating magnetic field of a prescribed frequency. The magnetic head is placed in opposition to condenser lens 6 with the magneto-optical disk 1 interposed therebetween.
The magneto-optical recordation-reproduction method of the aforementioned domain-enlarging reproduction system is explained below with reference to the timing charts shown in FIGS. 2A to 2H. Here, a method is explained for mark-edge recordation using a 1-7 code (RLL) as the memory modulation code.
FIG. 2A shows a channel clock for 1-7 modulation codes in recordation. FIG. 2B shows a sequence of 1-7 NRZI recording signals outputted from controller 24. FIG. 2C shows a sequence of record marks recorded by magnetic head 26. FIG. 2D shows heated regions at the clock timing. FIG. 2E shows application of the reproducing magnetic field controlled by controller 24. FIG. 2F shows reproduction signals corresponding to the record mark sequence shown in FIG. 2C.
FIG. 2G shows a reproduction clock signal. FIG. 2H shows reproduced binary signals corresponding to the record mark sequence shown in FIG. 2C.
The recordation is conducted as follows. Field modulation recordation is conducted by projecting a recording DC laser power in a pattern shown in FIG. 2B by LD driver 25 and semiconductor laser 7 onto a magneto-optical disk 1, corresponding to the channel clock shown in FIG. 2A, and applying a modulated magnetic field corresponding to the recordation signal by magnetic head driver 27 and driver 26. Thereby, a sequence of record marks shown in FIG. 2C are formed on the memory layer by changing the direction of the recording magnetization in accordance with the information. In FIG. 2C, the shadowed portions and the unshadowed portions show magnetic domains magnetized in directions reverse to each other.
The reproduction is conducted as follows. In the reproduction, a reproduction clock signal is formed in synchronization with the record pattern. The reproduction clock signal is formed in synchronization with the recording pattern, for example, by a sample-servo system by detecting synchronization signals of the record pattern from clock pits preliminarily formed on magneto-optical medium 1.
In the sample-servo system, the recordation is conducted also in synchronization with a clock signal formed by detection of the above clock pits. Therefore, by conducting the reproduction using the clock signal formed by detection of the clock pits, the recording and the reproduction can be synchronized with the clock signal.
Not only in the sample-servo system, a reproducing magnetic field can be applied in synchronization with the recorded domain sequence by conducting the recordation and reproduction by detecting the clock pits or the like formed preliminarily on the recording medium and conducting the recordation and the reproduction according to the clock signal.
In FIGS. 2A to 2H, the channel clock shown in FIG. 2A corresponds to the clock signal formed during recordation, and the signal shown in FIG. 2G corresponds to the clock signal for reproduction. In FIGS. 2A to 2H, the recordation phase and the reproduction phase are shifted by 180.degree. from each other. The reproducing magnetic field is applied in synchronization with the reproduction clock shown in FIG. 2G at the timing shown in FIG. 2E.
FIG. 2D shows the sampling time for judgement of the level of the reproduction signal, namely the heated regions on the recording medium heated by light projection for reproduction at the clock timing. The reproduction light is not limited to DC light and pulse light, but may be any light which is capable of forming heated regions to cause transfer of recording magnetic domains onto the readout layer.
A reproducing magnetic field having the same frequency as the modulation code channel clock of FIG. 2A is applied to these heated regions. Thereby, a recorded domain, which is present in the heated region in the memory layer, is transferred to the readout layer and is enlarged instantaneously by application of the magnetic field in a domain-enlarging direction to cause a sharp change in the regeneration signal.
Conversely, by reversing the direction of the applied alternating magnetic field, the enlarged magnetic domain is erased instantaneously to cause a sharp change of the reproduction signal (time (1) and time (2)). When no record mark is present in the memory layer, no magnetic domain is transferred to the readout layer, and the application of an alternating magnetic field does not cause enlargement of the magnetic domain, giving no change of reproduction signal (time (3) and time (4)). As a result, a reproduction signal is obtained as shown in FIG. 2F.
The reproduction signal shown in FIG. 2F obtained from differential amplification circuit 18 is binarized into binary data, and the obtained binary data is input to a D-input such as D-flip-flop 23. A reproduction clock signal formed in clock-forming phase switching circuit 21 is input to a clock input terminal. Thereby, a reproduced binary signal output is obtained as shown in FIG. 2H. The recorded data shown in FIG. 2B is reproduced as the signal shown in FIG. 2H.
As described above, the magneto-optical recording reproducing process enables recordation and reproduction of information. Further, the above described recording medium and the reproduction process employing the recording medium are not restricted by the resolution of the optical system, enabling an increase in record density.
However, in the aforementioned conventional magneto-optical mediums and magneto-optical processes, any recorded domain, which is present in the region having a coercivity force lowered by laser beam spot heating in the memory layer, will be transferred to the readout layer, and the transferred domain will be enlarged by the reproducing alternating magnetic field. Generally, the resolution during reproduction depends on the size of the heated region having a coercivity force lowered by laser beam irradiation.
In FIGS. 3A to 3H, FIG. 3A shows a channel clock signal of 1-7 modulation codes for recordation. FIG. 3B shows a sequence of 1-7 NRZI recording signals. FIG. 3C shows a record mark sequence recorded by magnetic head 26. FIG. 3D shows heated regions at clock timings. FIG. 3E shows a reproducing magnetic field controlled by controller 24. FIG. 3F shows a reproduced signal corresponding to the record mark sequence of FIG. 3D. FIG. 3G shows a reproducing clock signal. FIG. 3H shows reproduced binary signals intended to correspond to the record mark sequence of FIG. 3B.
A case is considered in which it is intended to record and reproduce signals in higher linear recordation density as shown in FIGS. 3A to 3H, for example. When reproducing no-record marks at time (3) and time (4) of the reproduction clock signal, recorded domains exist in heated regions (3) and (4), and the recorded domains are transferred to the memory layer and enlarged by application of the reproducing magnetic field.
Consequently, the no-record mark is reproduced as a record mark, which causes reproduction error, and the signal is reproduced as shown in FIG. 3F, and a reproduced binary signal sequence is obtained as shown in FIG. 3H, which is different from the recording signal sequence shown in FIG. 3B.
Since the size of the heated region is controlled by output intensity of the reproduction power, a reproduction signal may be obtained by lowering the output intensity of the reproduction power when high linear density of record marks is used. However, when considering the stability of the heated region, the decrease of the reproduction power output intensity is limited for the size decrease of the heated region.
From the aforementioned phenomenon, in reproduction of a record mark sequence by mark edge recordation regarding FIG. 3B, the length of one cycle of the channel clock signal should be larger than that of the heated region in the scanning direction so as not to transfer the record mark from the memory layer to the readout layer. Therefore, if the heated region is 0.2 .mu.m and the channel length is T, then T&gt;0.2 .mu.m. Therefore, the linear recordation density has an upper limit corresponding to the region of channel length T&gt;0.2 .mu.m. This is the recordation limit of the linear density in this type of recording medium and recording-reproducing process.